Leveling metal coatings



Feb. 10, 1959 x H. A. GAGE 2,

' LEVELING METAL COATINGS Filed Feb. 12. 1945 awuwvm Harold A. Gage LEVELXN G IVETAL COATINGS.

' Harold A. Gage, East Cleveland, Ohio, assignor to the United States of America as represented by the United States Atomic Energy Commission This invention relates to the application of metal coatings to cylindrical objects such as rods, tubes and the like by the hot dip method. it is especially concerned with improvements in the manipulation of such freshly-dipped objects to provide smooth uniform coatings.

In coating objects with molten metal by the hot dip process, it is necessary to cool the object below the melting point of the coating in some manner which will not injure or remove portions of the coating. This is especially true when it is desired to obtain a completely continuous coating to provide thorough protection of the object against corrosion. Suspension of the objects by means of racks during cooling leaves rack marks along which the coating has a difierent thickness and sometimes difierent physical properties from those of the coating at,

other points along the surface. For handling cylindrical objects it has been proposed to employ a supporting device comprising rotating discs upon which the object is supported. This method like the method employing racks leaves marks on the object where it is supported by the rotating discs.

It is an object of the present invention to provide uniform coatings on the entire surface of cylindrical objects so that they may be adequately protected not only along the entire peripheral surface but at both ends as well. Further objects will appear from the following general description and detailed illustrations of the invention.

In accordance with the present invention, cylindrical objects to whichmolten metal coatings have been applied by the hot dip method are placed in the trough formed by two parallel spaced rotating rollers having continuous, smooth surfaces and the objects are rotated by the rotation of the rollers until the coatings have solodified. By this procedure rods or cylinders may be provided with smooth even coatings without the presence of rack marks or other faults.

The rods or cylinders may be constructed of material having low thermal conductivity or low heat capacity or they may be provided with heating means to prevent a too rapid temperature drop. Carbon and steel rods have been employed with satisfactory results. An asbestos cement composition commercially available under the trade name Transite is especially suitable. Rolls of this material may be machined and burnished to provide a smooth surface.

The process of the invention is applicable to the coating of rods, pipes, and the like composed of any of the metals used in the manufacture of structural shapes, for example, iron, copper, nickel, lead, zinc, cadmium, tin, aluminum, and magnesium, and their alloys such as steel, bronze, brass, pewter, and the like, with the conventional coating metals, for example, zinc, lead, tin, cadmium, and aluminum, and their alloys such as bronze, brass, solder, zinc aluminum, aluminum-silicon and the like. The coating metal, of course, should be selected from among those metals or alloys having melting points sufiiciently below the melting point of the coated metal to permit their ap- States a ent plication without destruction of the article being coated.

The accompanying drawing illustrates one form of apparatus which may be used for accomplishing the purposes of the invention.

The apparatus illustrated comprises a metal frame 1 supporting a pair of polished asbestos cement composition rollers 2 and 3, spaced apart slightly and having sprockets 4 and 5 coupled to drive shaft 6 by chain 7 and sprocket 8.

In operating the apparatus the rate of rotation of the rollers can be varied widely. Surface speeds between 10 and 150 feet per minute are usually the most practicable. The cylindrical objects, after removal from a molten bath, are freed from excess metal and immediately placed in thetrough defined by the rollers 2 and 3 and allowed to remain for a time at least long enough for the coating to solidify. The numerals 9 and 10 designate objects so placed.

The following examples further illustrate the invention.

Example 1 Three solid metallic uranium rods, each 1.04 inches in diameter and 4 inches in length were pickled for 30 seconds in 50% I-INO solution and then were coatedby dipping in a molten metal bath comprising 94% zinc and 6% aluminum by weight at a temperature between 475 and 485 C. r

w The samples were removed from the molten metal bath and immediately placed in the trough formed by two carbon rods having a diameter of 5 /2 inches, spaced 1 inch apart and rotating in the same direction at 130-140 R. P. M. The uranium rods were rotated until the coatings had-solidified. They were then removed and examined The coatings were found to be smooth, even, and free fromsurface defects.

Example 2 A solid metallic uranium rod 1.1 inches in diameter and 4 inches in length was cleaned by dipping for20 seconds in aqueous 50% HnO solution containing 1% HCl and was then coated with zinc by dipping it through a flux comprising'30% lithium chloride, 50% potassium chloride and 20% sodium chloride into a molten bath of pure zinc at a temperature of 532 C. The rod was held in the moltenmetal for one minute during which the temperature of the metal dropped to 522 C.

The rod was removed from the bath, shaken briskly. to remove excess metal, and promptly placed in the trough formed by a pair of smooth Transite rollers, each 5 /2 inches in diameter, spaced inch from each other and rotating at a speed of 130-l40 R. P. M. The coated metal rod was rotated in this manner until the zinc coating had solidified. It was then cooled to normal temperature by quenching in water. The resulting coating was smooth and uniform throughout the length of the rod and over both ends of the rod.

Example 3 A metallic uranium rod cleaned by immersing for 5 minutes in 50% HNO at 65 C. was dipped through a flux of about 565 C. solidification point, prepared by adding to parts of flux of 53% KCl, 42% LiCl and 5% NaCl, 10 parts of NaF, and into a molten metal bath comprising 88% aluminum and 12% silicon at 620 C. After 3 minutes in the aluminum-silicon alloy bath the rod was withdrawn and rolled on cold steel rolls until the coating solidified. The rod was completely coated with a smooth, even coating of the aluminum alloy.

Example 4 An iron rod was pickled in an aqueous 12% HCl pickling bath for 5 minutes at room temperature. The rod egeeaaoe Y 3 was rinsed and then dipped for 5 minutes in a preflux of zinc chloride ammonium chloride eutectic towhich had been added KCl, 10% NaCl and enough NH Cl to raise the freezing point to 351) C. The flux temperature was. 4 Q Of C. I

"f'hefiux'coated rod was then dipped into "a molten bath of 2S grade'almnirmm at a temperature of 730 C. for 1 minute. 28 grade aluminum is a commercially pure wrought aluminum containing at least 99% aluminum and small amounts .of iron and silicon. The term is well known in 'the art and 'is precisely "defined in the Metals Handbook (T939 ed), pages 1246 and 1247.

The coated rod was withdrawn from the aluminum bath and rolled as inExampl'e '2 to solidify the coating. it was then quenched "in cold water. 'The iron rod was completely covered by'a metallic aluminum coating.

Example 5 An iron rod l hauing an electrodeposited coating or iron and over this an electrodeposited flash coating of copper was 'held in a 53% KCl, 42% LiCl, 5% KCl flux for 2 minutes and then in an underlying bath of 88 parts of aluminum and 12 parts of silicon at 600 C. for 60 seconds.

The rod was removed from the bath and rolled for 30 seconds as in Example 2 to solidify the coating, then quenched in water. The rod was completely covered 'by a coating of aluminum-silicon alloy. I

in each 'of'the examples set forth above, the rotation of the uranium rods was effected in an atmosphere of air at room temperature. The air was allowed to circulate freely so that the rods and rollers were exposed substant'ially continuously to an atmosphere at a temperature below the melting point of the coating material and were cooled by "this cooling medium "as well as by radiation. If desired, the rollers may he placed in a cooling chamber through which a cooling medium is circulated.

The feature of employing as a coating material an aluminum-containing zinc alloy "and the feature'o'f employing an alkali-metal halide are not claimed herein as .these features constitute the subject matter of U. S. patent applications, Serial Number 588,060 filed April 12, 1945, now 'PatentiNo. 2,848,796, granted August'26, 1958, by Lowell -D. Eubank and Ernest R. Boiler, and Serial Number 583,176 filed .March '16, 1945, now Patent No. 2,847,321, issued August 12, 1958 by Lowell ID. Eubank.

I-It will be understood that I intend to include variations and modifications of 'the invention and :that the preceding examples are illustrations only and in no wise to be construed .as limitations upon the invention, thefscope of which is defined in the appended claims, wherein I claim:

l. :l'n the application of metallic coatings to cylindrical uranium articles by hot dipping, the method of providing a level metal coating completely encasing said article which comprises dipping the article in a molten bath of a metal of the group consisting of zinc, lead, tin, cadmium, and aluminum and alloys thereof, placing the article while the coating is yet molten between a pair of driven, horizontal, closely spaced parallel rollers having smooth unyielding surfaces of low thermal conductivity extending at least the full length of said article, rotating said rollers in the same direction at substantially the same peripheral speed whereby the article is rotated and the molten metal coating smoothed, exposing said article to a cooling atmosphere at a temperature below the melting point of the coating metal, and continuing said rotation until the coating has solidified.

2. The method of coating a cylindrical uranium article thatcomprises cleaning the article by immersion for approximately five minutes in 5 3% nitric acid at a temperature of approximately '65" 'C., dipping the cleaned article'throug'h afiux prepared by adding 10% sodium fluoride to oi'a flux comprising approximately 53% potassium chloride, 42% lithium chloride, and 5% sodium chloride into a molten metal bath comprising approximate 88% aluminum and 12% silicon "at a temperature of approximately 620 for a period of approximately three minutes, withdrawing the coated article from the molten bath, placing the article while the coating is yet molten between a pair of driven, horizontal, closely spaced, parallel rollers lhaving smooth unyielding surfaces extending at least the full length of said article, rotating .saidrol'lers in the same direction at substantially the same peripheral speed whereby the article is rotated and the molten coating smoothed, exposing said article to a cooling atmosphere at a temperature below the melting point of 'thecoating metal, and continuing said rotation until the coating has solidified.

References Cited in the file of this patent UNITED STATES PATENTS 

1. IN THE APPLICATION OF METALLIC COATINGS TO CYLINDRICAL URANIUM ARTICLES BY HOT DIPPING, THE METHOD OF PROVINDING A LEVEL METAL COATING COMPLETELY ENCASING SAID ARTICLE WHICH COMPRISES DIPPING THE ARTICLE IN A MOLTERN BATH OF A METAL OF THE GROUP CONSISTING OF ZINC, LEAD, TIN, CADMIUM, AND ALUMINUM AND ALLOYS THEREOF, PLACING THE ARTICLE WHILE THE COATING IS YET MOLTEN BETWEEN A PAIR OF DRIVEN, HORIZONTAL, CLOSELY SPACED PARALLEL ROLLERS HAVING SMOOTH UNYIELDING SURFACES OF LOW THERMAL CONDUCTIVITY EXTENDING AT LEAST THE FULL LENGTH OF SAID ARTICLE, ROTATING SAID ROLLERS IN THE SAME DIRECTION AT SUBSTANTIALLY THE SAME PERIPHERAL SPEED WHEREBY THE ARTICLE IS ROTATED AND THE MOLTEN METAL COATING SMOOTHED,ESPOSING SAID ARTICLE TO A COOLING ATMOSPHERE AT A TEMPERATURE BELOW THE MELTING POINT OF THE COATING METAL,AND CONTINUING SAID ROTATION UNTIL THE COATING HAS SOLIDIFIED. 